4x19, Letters of Transit, is very important, considering Season 5 is a direct sequel, but there is something even more important:
In the middle of 4x14, The End of All Things, Peter goes into September's mind and gets some extra information about the timeline. Given the lengths the Observers have gone to in correcting September's mistake in 1985, and the.. strangeness.. about Etta, I have the feeling that this information will become important later on.
The finale, Brave New World parts one and two explain Walter's anger at William Bell during the events of Letters of Transit. They also contain an explanation for a blink-and-you'll-miss-it scene from 5x01 that otherwise looks like a small Deus ex Machina.
Then there's this quote from the Comic-Con trailer for Season 5:
Peter: "We need to keep those bald-headed bastards occupied."
Olivia: "What'd you have in mind?"
Peter: "I think it's time we caused a few Fringe events of our own."
Throughout the trailer are montages of various Fringe events throughout the series. The implication, as I understand it, is that they're going to take Fringe events from previous seasons and bring them to the next level - just like they did with Amber between its introduction in Season 1 and its usage to seal vortexes in later seasons.
I'm not saying you should make sure you've seen the entire series (Yes, I am), but I get the feeling that having at least an idea of what's happened in previous episodes will make Season 5 easier to understand, similar to how knowing Season 1 has given a lot more insight into events in Season 4.
This said, I do overall agree that it'll be hard to know what else is necessary until more of Season 5 has aired: Just look at what I've mentioned so far - I would not have thought of half it except that I've already seen 5x01.